With significant advances in inkjet technology in recent years, it has become possible for inkjet image forming devices to produce large color prints of high quality at high speeds. Such inkjet image forming devices are widely used particularly in sign and display applications, and are applicable, for example, to prints on POP (Point Of Purchase) posters, wall posters, outdoor advertisements, billboards, etc. Such inkjet image forming devices are capable of producing prints by forming a number of ink dots of several tens μm in diameter on a print medium by applying droplets of inks in different colors, e.g., C, M, Y, K inks, to the print medium.
It is known in the art that, according to human visual response characteristics, human color vision is most sensitive to shades of gray. More specifically, if dots printed in different colors are of the same size, then dots in a K colored ink can be perceived better visually than dots in other C, M, Y colored inks. Therefore, the observer of an image produced using an increased amount of K colored ink sees the image as grainy in its entirety (granularity is high), although the observer cannot identify the individual dots.
If dots printed in C, M, Y colored inks are placed closely together to form gray areas (composite black areas), then such dots provide a smoothing effect due to being superimposed on each other, thereby lowering image granularity. However, since three inks are used rather than one ink, the total amount of inks used increases, resulting in a high running cost (hereinafter referred to simply as an increased “cost”).
According to inkjet technology, therefore, there is a trade-off between granularity and cost, and it is important to design images so that granularity and cost of the images is well-balanced. Various color conversion techniques have been proposed in the art for appropriately controlling the total amount of inks used by finely adjusting the dot recording ratio while keeping the dots substantially isochromatic.
Japanese Laid-Open Patent Publication No. 2009-241609 discloses an apparatus for and a method of judging whether chromatic or achromatic dots are to be formed based on an input signal. The publication states that a pale K ink is used instead of a dark K ink in a color area where dot shapes are visible, thereby increasing granularity.
Japanese Laid-Open Patent Publication No. 10-044475 discloses an apparatus for and a method of correcting the amount of each recorded ink in order to increase the density of a certain ink (a Y ink among C, M, Y, K inks) the granularity of which is least visible.